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Liquefied natural gas (LNG) is natural gas which has been converted into liquid. This process is carried out because the substance is more easily transported or stored in liquid form than in gaseous form. Natural gas is converted into liquefied natural gas in a process which includes removal of contaminants followed by refrigeration. The process of liquefaction is carried out in a plant which contains process machinery referred to as an LNG train. A single plant usually contains multiple LNG trains, each capable of carrying out all the steps associated with conversion of natural gas to liquid.
Natural gas is composed primarily of methane, but also contains small traces of carbon dioxide, water, mercury, and hydrogen sulfide, as well as dust and other organic compounds. To convert the gas into a liquid, all of these contaminants must be removed, as they complicate the liquefaction process.
After contaminants have been removed, the gas is gradually refrigerated to convert it into a liquid. At -260 degrees Fahrenheit (-126.6 degrees Celsius), the gas condenses into liquefied natural gas. At this temperature, the liquid is condensed into around 1/600th of the volume of the gas. This significant reduction in volume means it is much more cost-efficient to transport the gas when it is in liquid form. This is particularly true in locations where natural gas pipeline networks are not in place.
Liquefied natural gas is transported in cryogenic road tankers or cryogenic sea-going carrier vessels. The term cryogenic refers to the fact that these vessels are equipped to maintain the temperature that keeps the gas liquid. Ships which transport the liquefied gas are built with double hulls to reduce the risk of damage to the hull which might cause leakage. The ships dock at LNG terminals, which have been specially constructed to deal with liquefied natural gas-transporting vessels. Terminals exist in Africa, Asia, North and South America, Australia, and a number of European countries.
There are several safety and environmental concerns associated with the conversion, storage, and transport of natural gas in liquid form. One of the most pressing is that large amounts of carbon dioxide are released when natural gas is extracted, and when it is converted into liquid. LNG plants also release nitrogen oxide and airborne particulates, which exacerbate the symptoms of respiratory diseases such as asthma and bronchitis.
Another issue is that natural gas is flammable, even when in liquid form. The liquid itself cannot combust, but if the liquid begins to vaporize and mix with air, the resulting gas is flammable. While there has never been a serious accident relating to LNG on board a transport vessel, there have been a number of accidents at LNG plants, including in the United States and Algeria. Many such accidents occur due to explosion of gas or liquid gas that vaporizes during the conversion process.